The invention relates to a gas-discharge lamp having brightness control, and particularly to a gas-discharge lamp including a circuit that provides duty-cycle shifting for brightness control.
It is desirable to control the intensity of a neon sign or other gas-discharge lamp application. This requires some sort of variable power source to drive the lamp. Neon power sources are typically one of two types: a neon transformer, or a neon power supply. A neon transformer steps up the utility voltage, and drives the neon lamps at utility frequency (50 or 60 Hz). A neon power supply rectifies the line voltage to form DC rail voltages, inverts the rail voltages at relatively high frequency (typically 20-100 kHz), and drives a small step up transformer that drives the tube. The present invention deals with a neon power supply.
Numerous methods have been used in an attempt to dim a neon lamp powered from a neon power supply. Some methods attempt to reduce the energy delivered to the tube on a continuous basis. One method includes reducing the DC rail voltages to the inverter. This and similar methods suffer from a common disadvantage; when dimmed, the center of large neon signs becomes dimmer than the sections electrically close to the incoming power. This is thought to result from capacitive losses along the length of the gas discharge tube.
One dimming method that gives the greatest range of dimming, with no significant difference in intensity along the length of the tube, is pulse group modulation (PGM, refer to FIG. 1). For PGM, the inverter is operated at full input voltage and optimum frequency (e.g., 20 kHz) for a first interval 15 of a time period 5 (i.e., a first group of pulses 10 is generated for a first interval 15). The inverter is then xe2x80x9cshut offxe2x80x9d for a second interval 25 of the time period 5 (i.e., no group of pulses 20 is generated in the second interval 25). The result is groups of drive pulses being delivered to the transformer and to the tube load. The on and off pulsing is continuously performed at a sufficiently high repetition rate to prevent the perception of flickering (about 100-200 Hz). The overall repetition rate is kept constant, while the lengths of the first and second intervals 15 and 25 are varied to implement dimming. The lamp is at full intensity when the ON interval 15 occupies the entire time period 5, and the lamp is off when the OFF interval 25 occupies the entire time period 5. In between lies a smooth range of dimming from off to fully bright. For a 200 Hz repetition rate and a 20 kHz drive frequency, it is possible to achieve 100 brightness steps, with good visual performance at all steps.
Pulse group modulation suffers from one major drawback. The step-up transformer oscillates at the pulse group repetition rate, producing a loud, annoying buzz. A subtler drawback of PGM dimming is that at lower brightness levels, the tube may extinguish and re-ignite with each pulse group. This continuous re-ionization generates radiation EMI.
One prior art method used to combat the above problems is frequency shift key (FSK) dimming (see FIG. 2). FSK dimming entails producing a first group of pulses 35 for a first interval 40 of a time period 45 (referred to as the xe2x80x9conxe2x80x9d portion or mode), ramping to a higher pulse frequency during a second interval 55, producing a second group of pulses 60 for a third interval 65 (referred to as the xe2x80x9coffxe2x80x9d portion or mode), and ramping down to the frequency of the first group of pulses 35 in a fourth interval 75. The transformer and tube are continuously driven, but with a much lower energy transfer efficiency during the xe2x80x9coffxe2x80x9d portion 60. By varying the amount of time spent in the normal high efficiency xe2x80x9conxe2x80x9d mode 45 and the low efficiency xe2x80x9coffxe2x80x9d mode 55, the sign can be progressively dimmed. Also, since the transformer is continuously driven, the audible noise generated by the pulse group repetition is dramatically reduced.
FSK dimming suffers from one major drawback. The continuously changing drive frequencies generate a wide spectrum of electromagnetic interference (EMI) noise, making EMI filtering difficult. However, since FSK dimming continuously drives the tube, it is always ignited, and re-ignition radiated EMI is not a concern.
Accordingly, in one embodiment, the invention provides a gas-discharge lamp connectable to a power source and to a gas-discharge tube for controlling brightness of the tube. The lamp includes a drive having first and second switches. The drive is configured to receive direct current (DC) power, receive control signals, and invert the DC power to create a first varying signal in response to the control signals. The lamp further includes a transformer interconnected to the drive. The transformer transforms the first varying signal to a second varying signal; the second varying signal is supplied to the tube. The lamp further includes a controller interconnected to the drive. The controller generates the control signals for a time period and provides the control signals to the first and second switches. The generating of the control signals includes for a first interval of the time period, generating a first control signal with a first duty cycle, the first control signal being provided to the first switch, and generating a second control signal with a second duty cycle, the second control signal being provided to the second switch, and, for a second interval of the time period, generating a third control signal with a third duty cycle, the third control signal being provided to the first switch, and generating a fourth control signal with a fourth duty cycle, the fourth control signal being provided to the second switch. The third duty cycle is less than the first duty cycle, and the fourth duty cycle is less than the second duty cycle. The generation of the control signals just described is referred to herein as duty-cycle shifting (DCS).
The invention also provides a method of controlling the brightness of a gas-discharge lamp including a power supply. The power supply includes a drive having first and second switches. The drive supplies a varying signal in response to receiving control signals. The method includes establishing a time period; for a first interval of the time period, generating a first control signal having a first duty cycle and providing the first control signal to the first switch, and generating a second control signal having a second duty cycle and providing the second control signal to the second switch; and, for a second interval of the time period, generating a third control signal having a third duty cycle and providing the third control signal to the first switch, and generating a fourth control signal having a fourth duty cycle and providing the fourth control signal to the second switch. The third duty cycle is less than the first duty cycle, and the fourth duty cycle is less than the second duty cycle.
Duty-cycle shifting, like pulse group modulation, shares the advantage of a very large dynamic range. The neon sign can be dimmed from full brightness down to a very low intensity. This is accomplished without some of the undesirable effects of prior art dimming methods. For example, duty-cycle shifting prevents uneven dimming along the length of the tube, and prevents extinguishing or de-ionization of the tube. Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings.